MMP1 and rheumatoid arthritis: Studies performed by Alsalameh and colleagues [31] on synovial fibroblasts from patients with rheumatoid arthritis and patients with osteoarthritis have also indicated there is a differential expression of the two TNF receptors (TNF-R1 and TNF-R2) in these cells and that, although both receptors can mediate the effect of TNF-α on TIMP1 expression, PGE2, the regulation of IL-6, and MMP-1 is mediated exclusively by TNF-R1.